Lithium metal electrodes and batteries thereof

ABSTRACT

The present disclosure is generally related to separators for use in lithium metal batteries, and associated systems and products. Certain embodiments are related to separators that form or are repaired when an electrode is held at a voltage. In some embodiments, an electrochemical cell may comprise an electrolyte that comprises a precursor for the separator.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 62/318,470, filed Apr. 5, 2016and entitled “Lithium Metal Electrodes and Batteries Thereof,” which isincorporated herein by reference in its entirety for all purposes.

FIELD

The present invention relates generally to systems and methods forforming separators in electrochemical cells and healing defects inseparators in electrochemical cells.

BACKGROUND

Lithium metal batteries are a promising technology because of the highspecific energy of lithium. However, many lithium metal batteriesexperience premature failure due to dendrite growth from the anode tothe cathode. Separators have been added to lithium metal batteries toarrest dendrite growth, but often use their utility once damaged bygrowing dendrites.

Accordingly, improved compositions and methods are desirable.

SUMMARY

Methods and articles for formation and healing of separators inelectrochemical cells are generally provided. The subject matter of thepresent invention involves, in some cases, interrelated products,alternative solutions to a particular problem, and/or a plurality ofdifferent uses of one or more systems and/or articles.

In one aspect, methods of forming separators in rechargeableelectrochemical cells are generally provided. A method may compriseholding a first electrode at a first voltage in a rechargeableelectrochemical cell. The electrochemical cell may comprise anelectrolyte comprising a precursor for the separator in an amount ofless than or equal to 1 mM and greater than or equal to 1 nM. In someembodiments, the first voltage causes the precursor for the separator toreact to form a separator positioned between the first electrode and asecond electrode.

In another aspect, methods of healing defects in separators inrechargeable electrochemical cells are provided. A method may compriseholding a first electrode at a first voltage in a rechargeableelectrochemical cell. The rechargeable electrochemical cell may comprisea separator and an electrolyte comprising a precursor for the separatorin an amount of less than or equal to 1 mM and greater than or equal to1 nM. In some embodiments, the first voltage may cause the precursor forthe separator to react to heal a defect in the separator.

Some embodiments are related to rechargeable electrochemical cells. Inone embodiment, a rechargeable electrochemical cell comprises a firstelectrode, a second electrode, and an electrolyte. The electrolyte maycomprise a precursor for a separator that has a solubility in theelectrolyte of less than or equal to 1 mM and greater than or equal to 1nM.

In some embodiments, a rechargeable electrochemical cell comprises afirst electrode, a second electrode, and an electrolyte. The electrolytemay comprise at least one of a first halide anion and a species that canreact to form a first halide anion, and comprises at least one of asecond halide anion and a species that can react to form a second halideanion.

In some embodiments, a rechargeable electrochemical cell comprises afirst electrode, a second electrode, and a separator. The separator maycomprise at least a first layer and a second layer, and the second layermay undergo oxidation at a higher voltage than the first layer.

Other advantages and novel features of the present invention will becomeapparent from the following detailed description of various non-limitingembodiments of the invention when considered in conjunction with theaccompanying figures. In cases where the present specification and adocument incorporated by reference include conflicting and/orinconsistent disclosure, the present specification shall control. If twoor more documents incorporated by reference include conflicting and/orinconsistent disclosure with respect to each other, then the documenthaving the later effective date shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIG. 1 shows a rechargeable electrochemical cell comprising a firstelectrode, a second electrode, and an electrolyte comprising a precursorfor a separator, according to some embodiments;

FIG. 2 shows a rechargeable electrochemical cell comprising a firstelectrode, a second electrode, an electrolyte comprising a precursor fora separator, and a separator, according to some embodiments;

FIG. 3 shows a rechargeable electrochemical cell comprising a firstelectrode, a second electrode, an electrolyte, and a separator,according to some embodiments;

FIG. 4 shows a rechargeable electrochemical cell comprising a firstelectrode, a second electrode, an electrolyte, and a separatorcomprising a first layer and a second layer, according to someembodiments;

FIG. 5A shows a rechargeable electrochemical cell comprising a firstelectrode, a second electrode, an electrolyte comprising a precursor fora separator, and a separator comprising a defect, in accordance withsome embodiments;

FIG. 5B shows a method for repairing a defect in a rechargeableelectrochemical cell, according to some embodiments;

FIG. 6 shows a method for forming a lithium halide layer, in accordancewith some embodiments;

FIG. 7 is a plot showing various properties of various lithium halidesalts, in accordance with some embodiments;

FIG. 8 shows self-healing in separators, according to some embodiments;

FIG. 9 shows a method for forming a passivation layer on an electrode,according to some embodiments;

FIG. 10 shows a method for forming a separator, according to someembodiments;

FIG. 11 shows calculated values of adsorption and formation energy forlithium halide salts, according to some embodiments;

FIG. 12 shows a solution phase diagram for I⁻/I₂/I₃ ⁻, according to someembodiments;

FIGS. 13A-C show the measured impedance for various electrochemicalcells, according to some embodiments; and

FIGS. 14A-D show cyclic voltammetry measurements performed on variouselectrochemical cells, according to some embodiments.

DETAILED DESCRIPTION

Systems and methods related to the formation and/or the repair ofseparators in rechargeable electrochemical cells are generally provided.Certain articles and methods relate to forming and/or repairing aseparator in a rechargeable electrochemical cell in situ. In someembodiments, a rechargeable electrochemical cell may lack a separatorafter assembly but a separator may form during electrochemical cellcharging and/or discharging. In some embodiments, a rechargeableelectrochemical cell may comprise an ex situ separator upon assembly andan in situ separator may form during electrochemical cell chargingand/or discharging. Separators, such as those formed duringelectrochemical cell charging and/or discharging, may comprise one ormore defects that may be repaired by one or more species present in theelectrochemical cell.

Certain embodiments relate to rechargeable electrochemical cells thatcomprise one or more precursors for a separator. The precursor for theseparator may be a species that is capable of reacting to form aseparator in an electrochemical cell and/or to repair a defect in aseparator in an electrochemical cell. The precursor for the separatormay, in some embodiments, react to form a separator and/or to repair adefect in a separator when one or more electrodes are held at a voltagein the electrochemical cell. In some embodiments, a precursor for anelectrochemical cell may comprise a halide or a species that comprises ahalide. In some embodiments, a precursor for a separator may have arelatively low solubility in an electrolyte present in the rechargeableelectrochemical cell, and/or may be present at a relatively lowconcentration in an electrolyte present in the rechargeableelectrochemical cell. As used herein, “separator” is given its ordinarymeaning in the art. In one set of embodiments it is a solid or gelmaterial that physically separates an anode from a cathode and preventsshorting. A precursor of a separator is a substance which, by itself, isnot as effective as a separator but, upon holding one of the electrodesat a set voltage, or cycling the electrode, or other implementationevent as described herein, forms a separator.

As described above, certain embodiments relate to rechargeableelectrochemical cells. FIG. 1 shows one non-limiting embodiment of arechargeable electrochemical cell 100 that comprises first electrode110, second electrode 120, and electrolyte 130 comprising precursor fora separator 140. Although FIG. 1 shows one precursor for a separator, itshould be understood that in some embodiments two, three, four, or moreprecursors for a separator may also be present in the electrolyte.

In some embodiments, a rechargeable electrochemical cell may comprise aprecursor for a separator that may undergo a reaction to form aseparator. For example, holding a first electrode at a first voltage ina rechargeable electrochemical cell may result in the formation of aseparator. The reaction may be any suitable reaction, such as a redoxreaction, a polymerization reaction, and/or crystallization reaction. Insome embodiments, a reaction may comprise the crystallization on anelectrochemical cell component (e.g., on a first electrode) of a solutedissolved in the electrolyte. The first electrode may be held at thefirst voltage by any suitable means. In some embodiments, a voltage maybe applied to the first electrode, such as, for example, by an externalvoltage source. In some embodiments, the first voltage may be a voltagethat the first electrode inherently has when it is positioned in therechargeable electrochemical cell. In some embodiments, the firstvoltage is greater than or equal to 0 V, greater than or equal to 1.5 V,greater than or equal to 2 V, greater than or equal to 2.5 V, greaterthan or equal to 3 V, greater than or equal to 3.5 V, greater than orequal to 4 V, greater than or equal to 4.5 V, greater than or equal to 5V, or greater than or equal to 5.5 V. In some embodiments, the firstvoltage is less than or equal to 6 V, less than or equal to 5.5 V, lessthan or equal to 5 V, less than or equal to 4.5 V, less than or equal to4 V, less than or equal to 3.5 V, less than or equal to 3 V, less thanor equal to 2.5 V, less than or equal to 2 V, less than or equal to 1.5V, less than or equal to 1 V, or less than or equal to 0.5 V.Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 0 V and less than or equal to 6 V). Otherranges are also possible. The first voltage should be understood to bethe voltage with respect to a Li⁺/Li reference potential.

FIG. 2 shows one embodiment in which precursor for a separator 140undergoes a reaction to form separator 150 in rechargeableelectrochemical cell 100. In some embodiments, such as that shown inFIG. 2, the separator may form directly on the first electrode. However,in other embodiments the separator may not form directly on the firstelectrode. For example, the separator may form on one or moreintervening electrochemical cell components (e.g., one or morepassivation layers, which will be described further below) disposed onthe first electrode. In some embodiments, the separator may form(directly or indirectly) on the first electrode and on one or more ofthe second electrode, the electrolyte, and an ex situ separator. In someembodiments, one layer of a separator (e.g., a second layer) may form ona first layer (which either formed in situ or ex situ). Otherconfigurations are also possible.

A rechargeable electrochemical cell component referred to as being“disposed on,” “disposed between,” “on,” or “adjacent” anotherrechargeable electrochemical cell component(s) means that it can bedirectly disposed on, disposed between, on, or adjacent the rechargeableelectrochemical cell component (s), or an intervening rechargeableelectrochemical cell component may also be present. A rechargeableelectrochemical cell component that is “directly adjacent,” “directlyon,” or “in contact with,” another rechargeable electrochemical cellcomponent means that no intervening electrochemical cell component ispresent. It should also be understood that when a rechargeableelectrochemical cell component is referred to as being “disposed on,”“disposed between,” “on,” or “adjacent” another rechargeableelectrochemical cell component(s), it may be covered by, on or adjacentthe entire rechargeable electrochemical cell component(s) or a part ofthe rechargeable electrochemical cell component(s).

In some embodiments, a rechargeable electrochemical cell as describedherein may initially lack a separator formed in situ, but may form aseparator in situ during electrochemical cell cycling. In someembodiments, the separator may form during the first cycle, aftergreater than or equal to 1 cycle, greater than or equal to 2 cycles, orgreater than or equal to 3 cycles. In some embodiments, the separatormay form after less than or equal to 4 cycles, less than or equal to 3cycles, less than or equal to 2 cycles, or less than or equal to 1cycle. Combinations of the above-referenced ranges are also possible(e.g., during the first cycle and after less than or equal to 4 cycles).Other ranges are also possible. The formation of the separator may bedetermined by using electron microscopy. Without wishing to be bound bytheory, it is believed that the rate of separator formation may beaffected by one or more of the following factors: the identity and/orconcentration of the precursor, the rate of cycling, the temperature atwhich cycling occurs, the voltage limits present during cycling, thecomposition of an electrolyte present during cycling, the identityand/or concentration of additives present during cycling, theelectroactive material in the first and/or second electrodes, theidentity of conductivity additives in the first and/or second electrode,the identity of a binder in the first and/or second electrodes, and theidentity of any current collectors.

As described above, certain embodiments relate to electrochemical cellsthat comprise separators, such as electrochemical cells that compriseseparators that are formed in situ. FIG. 3 shows one example of anelectrochemical cell that includes a separator, where rechargeableelectrochemical cell 100 includes first electrode 110, second electrode120, electrolyte 130, and separator 150. In some, but not necessarilyall, embodiments, the separator is a separator that has formed in situ.The electrolyte may comprise a precursor for a separator, and theprecursor for the separator may be a precursor for the separator presentin the rechargeable electrochemical cell (i.e., it may be capable ofundergoing a reaction to form a separator with substantially the samecomposition as the separator present in the electrochemical cell), or itmay be a precursor for a separator different from the separator presentin the rechargeable electrochemical cell (i.e., it may be capable ofundergoing a reaction to form a separator with a different compositionthan the separator present in the rechargeable electrochemical cell). Insome embodiments, the rechargeable electrochemical cell comprises anelectrolyte that does not include a precursor for a separator.

In some embodiments, such as that shown in FIG. 3, the separator may bedirectly adjacent the first electrode. However, in other embodiments theseparator may not be directly adjacent the first electrode. For example,the separator may be adjacent one or more intervening electrochemicalcell components (e.g., one or more passivation layers, which will bedescribed further below) disposed on the first electrode. In someembodiments, the separator may be directly or indirectly adjacent thefirst electrode and one or more of the second electrode, theelectrolyte, and an ex situ separator. Other configurations are alsopossible.

In some embodiments, a rechargeable electrochemical cell may comprise aseparator including more than one layer. FIG. 4 shows one non-limitingembodiment of a rechargeable electrochemical cell which includes aseparator with two layers. In FIG. 4, rechargeable electrochemical cell100 comprises first electrode 110, second electrode 120, electrolyte130, and separator 150. Separator 150 comprises first layer 152 andsecond layer 154. In some embodiments, the first layer may be positionedcloser to the first electrode than the second layer. The second layer ofthe separator may have a substantially similar composition to the firstlayer of the separator, or the two layers may have differentcompositions. In some cases, the second layer of the separator mayundergo oxidation at a higher voltage than the first layer. For example,the separator may include a first layer that comprises LiI (which maymake up any suitable wt % of the first layer up to 100 wt %) and asecond layer that comprises LiF (which may make up any suitable wt % ofthe second layer up to 100 wt %). Without wishing to be bound by theory,it may be advantageous for the outermost layer of the separator to bethe layer within the separator that undergoes oxidation at the highestvoltage because it may result in a separator in which the outermostlayer is the layer that is most stable. The stable top layer may preventerosion and/or destruction of underlying layers that are less stable. Itmay also be advantageous for the first layer to have a high ionconductivity and/or for the second layer to have a relatively lowsolubility in the electrolyte (e.g., between 1 nM and 1 mm).

In some embodiments, a precursor for an electrochemical cell maycomprise at least a first layer and a second layer, and the second layermay undergoes oxidation at a voltage that is greater than or equal to 3%higher than the voltage at which the first layer undergoes oxidation, avoltage that is greater than or equal to 5% higher than the voltage atwhich the first layer undergoes oxidation, a voltage that is greaterthan or equal to 10% higher than the voltage at which the first layerundergoes oxidation, a voltage that is greater than or equal to 15%higher than the voltage at which the first layer undergoes oxidation, avoltage that is greater than or equal to 20% higher than the voltage atwhich the first layer undergoes oxidation, a voltage that is greaterthan or equal to 25% higher than the voltage at which the first layerundergoes oxidation, a voltage that is greater than or equal to 30%higher than the voltage at which the first layer undergoes oxidation, avoltage that is greater than or equal to 35% higher than the voltage atwhich the first layer undergoes oxidation, a voltage that is greaterthan or equal to 40% higher than the voltage at which the first layerundergoes oxidation, a voltage that is greater than or equal to 45%higher than the voltage at which the first layer undergoes oxidation, ora voltage that is greater than or equal to 50% higher than the voltageat which the first layer undergoes oxidation, or a voltage that isgreater than or equal to 55% higher than the voltage at which the firstlayer undergoes oxidation. In some embodiments, the second layerundergoes oxidation at a voltage that is less than or equal to 60%higher than the voltage at which the first layer undergoes oxidation, avoltage that is less than or equal to 55% higher than the voltage atwhich the first layer undergoes oxidation, a voltage that is less thanor equal to 50% higher than the voltage at which the first layerundergoes oxidation, a voltage that is less than or equal to 45% higherthan the voltage at which the first layer undergoes oxidation, a voltagethat is less than or equal to 40% higher than the voltage at which thefirst layer undergoes oxidation, a voltage that is less than or equal to35% higher than the voltage at which the first layer undergoesoxidation, a voltage that is less than or equal to 30% higher than thevoltage at which the first layer undergoes oxidation, a voltage that isless than or equal to 25% higher than the voltage at which the firstlayer undergoes oxidation, a voltage that is less than or equal to 20%higher than the voltage at which the first layer undergoes oxidation, avoltage that is less than or equal to 15% higher than the voltage atwhich the first layer undergoes oxidation, a voltage that is less thanor equal to 10% higher than the voltage at which the first layerundergoes oxidation, or a voltage that is less than or equal to 5%higher than the voltage at which the first layer undergoes oxidation.Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 3% higher and less than or equal to 60%higher). Other ranges are also possible. The voltage at which a layerundergoes oxidation may be determined by cyclic voltammetry.

Although FIG. 4 shows a rechargeable electrochemical cell including aseparator with only two layers, it should be understood that separatorsmay comprise more than two layers. In some embodiments, a separatorcomprises greater than or equal to three layers, greater than or equalto four layers, or even more layers. Each layer within the separator mayhave a substantially similar composition to each other layer in theseparator, or one or more layers within the separator may have adifferent composition than one or more other layers within theseparator.

As described above, certain embodiments relate to methods for healing adefect within a separator. The separator may be an in situ separator, ormay be an ex situ separator. One example of a method for healing adefect is shown in FIGS. 5A and 5B. FIG. 5A shows rechargeableelectrochemical cell comprising first electrode 110, second electrode120, electrolyte 130 comprising precursor for the separator 140, andseparator 150 comprising defect 160. In FIG. 5B, precursor for theseparator 140 undergoes a reaction to heal defect 160 so that theseparator is no longer damaged, or damaged to a smaller degree. Incertain embodiments, one or more defect in the separator may be healedby the formation of a material in the defect with a substantiallysimilar composition to the separator, as is shown in FIG. 5B. In otherembodiments, a defect in the separator may be healed by the formation ofa material in the defect with a different composition than theseparator. Non-limiting examples of defects that may be healed includecracks, pits, pinholes, and the like.

In some embodiments, a defect in a separator may be healed by holdingthe first electrode at a first voltage. In some embodiments, the avoltage may be applied to the first electrode, such as, for example, byan external voltage source. In some embodiments, the first voltage maybe a voltage that the first electrode inherently has when it ispositioned in the rechargeable electrochemical cell. In someembodiments, the first voltage is greater than or equal to 0 V, greaterthan or equal to 1.5 V, greater than or equal to 2 V, greater than orequal to 2.5 V, greater than or equal to 3 V, greater than or equal to3.5 V, greater than or equal to 4 V, or greater than or equal to 4.5 V.In some embodiments, the first voltage is less than or equal to 5 V,less than or equal to 4.5 V, less than or equal to 4 V, less than orequal to 3.5 V, less than or equal to 3 V, less than or equal to 2.5 V,less than or equal to 2 V, less than or equal to 1.5 V, less than orequal to 1 V, or less than or equal to 0.5 V. Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 0 V and less than or equal to 5 V). Other ranges are also possible.The first voltage should be understood to be the voltage with respect toa Li⁺/Li reference potential.

In some embodiments, a precursor for a separator as described herein mayhave one or more properties that are beneficial for an electrochemicalcell. For instance, a rechargeable electrochemical cell may comprise aprecursor for the separator that does not participate in a redox shuttlemechanism when the rechargeable electrochemical cell operates at apotential of greater than or equal to 2.5 V, greater than or equal to 3V, 3.5 V, greater than or equal to 4 V, greater than or equal to 4.5 V,or greater than or equal to 5 V. Without wishing to be bound by theory,a redox shuttle mechanism is a process that happens in certainelectrochemical cells in which a species is oxidized at the cathode,diffuses to the anode, and then is reduced at the anode. This results inan internal short circuit, and decreases the amount of power provided byand the roundtrip efficiency of the rechargeable electrochemical cell.

In some embodiments, a rechargeable electrochemical cell may comprise aprecursor for a separator that is relatively insoluble in an electrolytethat is also present in the rechargeable electrochemical cell. Theprecursor for the separator may have a solubility in the electrolyte ofless than or equal to 1 mM, less than or equal to 100 μM, less than orequal to 10 μM, less than or equal to 1 μM, less than or equal to 100nM, or less than or equal to 10 nM. The precursor for the separator mayhave a solubility in the electrolyte of greater than or equal to 1 nM,greater than or equal to 10 nM, greater than or equal to 100 nM, greaterthan or equal to 1 greater than or equal to 10 or greater than or equalto 100 μM. Combinations of the above-referenced ranges are also possible(e.g., greater than or equal to 1 nM and less than or equal to 1 mM).Other ranges are also possible.

In some embodiments, a rechargeable electrochemical cell may comprise aprecursor for a separator that is present in an electrolyte that is alsopresent in the rechargeable electrochemical cell at a relatively lowconcentration. In some embodiments, the precursor for the separator maybe present in the electrolyte at a concentration of less than or equalto 1 mM, less than or equal to 100 less than or equal to 10 less than orequal to 1 less than or equal to 100 nM, or less than or equal to 10 nM.The precursor for the separator may be present in the electrolyte at aconcentration of greater than or equal to 1 nM, greater than or equal to10 nM, greater than or equal to 100 nM, greater than or equal to 1greater than or equal to 10 or greater than or equal to 100 μM.Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 1 nM and less than or equal to 1 mM). Otherranges are also possible.

In certain embodiments, a precursor for a separator described herein maycomprise a species with one or more anions, such as a salt. In someembodiments, the precursor for the separator comprises a salt dissolvedin an electrolyte. The precursor for the separator may comprise one ormore halide anions, such as one or more of a fluoride anion, a chlorideanion, a bromide anion, and an iodide anion. In some embodiments, theprecursor for the separator may comprise at least two halide anions, atleast three halide anions, or more halide anions. In some embodiments,the precursor for the separator may comprise an anion that comprises oneor more halogen atoms, such as a polyhalide anion. When present, thepolyhalide anion may comprise only one type of halogen atom (such as,e.g., I₃ ⁻, which only comprises iodine) or ma may comprise two or moretypes of halogen atoms (such as, e.g., ClBr₂ ⁻). In some embodiments,the precursor for the separator may comprise a species that may undergoa reaction (e.g., a redox reaction, a chemical reaction) to form ahalide anion and/or a polyhalide anion. Non-limiting examples of suchspecies include LiPF₆ and LiBF₄.

In some embodiments, a precursor for a separator may comprise a speciesthat does not comprise any halogen atoms. For example, the precursor forthe separator may comprise a salt dissolved in an electrolyte thatcomprises an anion that does not include any halogen atoms. Non-limitingexamples of such anions include chlorate anions, perchlorate anions,nitrate anions, phosphate anions, bis(fluorosulfonyl)imide anions, andbis(trifluoromethane)sulfonimide anions.

As described above, in some embodiments a precursor for a separator maycomprise a salt, such as a salt dissolved in an electrolyte.Non-limiting examples of suitable cations for the salt include alkalimetal cations such as lithium and sodium, alkaline earth metal cationssuch as magnesium, and transition metal cations such as zinc and copper.

In some embodiments, a rechargeable electrochemical cell as describedherein may comprise a precursor for a separator that is not a salt or acomponent of a salt. For example, the rechargeable electrochemical cellmay comprise a precursor for a separator that is a halogen with anoxidation state of zero, such as I₂.

In some embodiments, a rechargeable electrochemical cell may comprise atleast two precursors for a separator, at least three precursors for aseparator, or more precursors for a separator. In some embodiments, twoor more of the precursors for the separator may be species that arehalide anions or can react to form halide anions. For example, anelectrochemical cell may comprise at least a first precursor for aseparator that is at least one of a first halide anion and a speciesthat can react to form a first halide anion and a second precursor for aseparator that is at least one of a second halide anion and a speciesthat can react to form a second halide anion. In some embodiments, therechargeable electrochemical cell may comprise at least a firstprecursor for a separator that is at least one of a first halide anionand a species that can react to form a first halide anion and a secondprecursor for a separator that is not a halide anion or a species thatcan react to form a halide anion.

As described above, certain embodiments relate to rechargeableelectrochemical cells that comprise a separator. The separator may be asolid species that prevents the first electrode from contacting thesecond electrode, or a species that prevents or significantly retardsthe formation of a short circuit from the first electrode to the secondelectrode. In some embodiments, the separator may be a single ionconductor, or may be a solid electrolyte. For example, it may be capableof conducting cations but not anions. In some embodiments, the separatormay have a relatively high ionic conductivity. For instance, the ionicconductivity of the separator may be greater than or equal to 10⁻⁴/cm,greater than or equal to 10⁻³ S/cm, greater than or equal to 10⁻² S/cm,greater than or equal to 10⁻¹ S/cm, greater than or equal to 10⁰ S/cm,or greater than or equal to 10¹ S/cm. In some embodiments, the ionicconductivity of the separator may be less than or equal to 10² S/cm,less than or equal to 10¹ S/cm, less than or equal to 10⁰ S/cm, lessthan or equal to 10⁻¹ S/cm, less than or equal to 10⁻² S/cm, or lessthan or equal to 10⁻³ S/cm. Combinations of the above-referenced rangesare also possible (e.g., greater than or equal to 10⁻⁴ S/cm and lessthan or equal to 10² S/cm, greater than or equal to 10⁻⁴ S/cm and lessthan or equal to 10⁻² S/cm). Other ranges are also possible. The ionicconductivity of the separator may be determined by electrochemicalimpedance spectroscopy.

In some embodiments, a rechargeable electrochemical cell may comprise aseparator with a relatively low area-specific impedance. In someembodiments, the area-specific impedance of the separator may be lessthan or equal to 100 Ohm*cm², less than or equal to 50 Ohm*cm², lessthan or equal to 20 Ohm*cm², less than or equal to 10 Ohm*cm², less thanor equal to 5 Ohm*cm², less than or equal to 2 Ohm*cm², or less than orequal to 1 Ohm*cm². In some embodiments, the area-specific impedance ofthe separator may be greater than or equal to 0.5 Ohm*cm², greater thanor equal to 1 Ohm*cm², greater than or equal to 2 Ohm*cm², greater thanor equal to 5 Ohm*cm², greater than or equal to 10 Ohm*cm², greater thanor equal to 20 Ohm*cm², or greater than or equal to 50 Ohm*cm².Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 0.5 Ohm*cm² and less than or equal to 100Ohm*cm², greater than or equal to 0.5 Ohm*cm² and less than or equal to20 Ohm*cm²). The area-specific impedance of the separator may bedetermined by electrochemical impedance spectroscopy.

In some embodiments, a rechargeable electrochemical cell may comprise aseparator that is stable at a relatively high voltage. As used herein, aseparator is considered to be stable at a voltage if it does not undergoappreciable degradation such as dissolution, cracking, oxidation, andthe like, when held at that voltage. In some embodiments, the separatormay be stable at a voltage of greater than or equal to 2 V, greater thanor equal to 2.5 V, greater than or equal to 3 V, 3.7 V, greater than orequal to 4.0 V, greater than or equal to 4.2 V, greater than or equal to4.3 V, greater than or equal to 4.5 V, or greater than or equal to 4.7V. In some embodiments, the separator may be stable at a voltage of lessthan or equal to 5 V, less than or equal to 4.7 V, less than or equal to4.5 V, less than or equal to 4.3 V, less than or equal to 4.2 V, lessthan or equal to 4.0 V, less than or equal to 3.7 V, less than or equalto 3 V, or less than or equal to 2.5 V. Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 2 V and less than or equal to 5 V, greater than or equal to 3.7 V andless than or equal to 5 V). Other ranges are also possible.

In some embodiments, a rechargeable electrochemical cell may comprise aseparator with a relatively high shear modulus. Without wishing to bebound by theory, it is believed that separators that have a high shearmodulus may be more resistant to dendrite growth and so may improve thecycle life of the rechargeable electrochemical cell. In someembodiments, the separator has a shear modulus of greater than or equalto 5 GPa, greater 7.5 GPa, greater than or equal to 10 GPa, greater thanor equal to 12.5 GPa, greater than or equal to 15 GPa, or greater thanor equal to 17.5 GPa. In some embodiments, the separator has a shearmodulus of less than or equal to 20 GPa, less than or equal to 17.5 GPa,less than or equal to 15 GPa, less than or equal to 12.5 GPa, less thanor equal to 10 GPa, or less than or equal to 5 GPa. Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 5 GPa and less than or equal to 20 GPa). Other ranges are alsopossible. The shear modulus of the separator may be determined by pulseecho ultrasonic methods.

In some embodiments, a rechargeable electrochemical cell may comprise aseparator with a relatively low linear coefficient of thermal expansion.The linear coefficient of thermal expansion may be less than or equal to0.0000105 K⁻¹, less than or equal to 10⁻⁵ K⁻¹, or less than or equal to10′ K⁻¹. The linear coefficient of thermal expansion may be determinedby thermomechanical analysis.

A separator may have any suitable thickness. In some embodiments, thethickness of the separator is greater than or equal to 1 nm, greaterthan or equal to 2 nm, greater than or equal to 5 nm, greater than orequal to 10 nm, greater than or equal to 20 nm, greater than or equal to50 nm, greater than or equal to 100 nm, greater than or equal to 200 nm,greater than or equal to 500 nm, greater than or equal to 1 micron,greater than or equal to 2 microns, greater than or equal to 5 microns,greater than or equal to 10 microns, or greater than or equal to 20microns. In some embodiments, the thickness of the separator is lessthan or equal to 50 microns, less than or equal to 20 microns, less thanor equal to 10 microns, less than or equal to 5 microns, less than orequal to 2 microns, less than or equal to 1 micron, less than or equalto 500 nm, less than or equal to 200 nm, less than or equal to 100 nm,less than or equal to 50 nm, less than or equal to 20 nm, less than orequal to 10 nm, less than or equal to 5 nm, or less than or equal to 2nm. Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 1 nm and less than or equal to 50 microns, orgreater than or equal to 10 nm and less than or equal to 1 micron).Other ranges are also possible. The thickness of the separator may bedetermined by electron microscopy.

The separator may have any suitable morphology and composition. In someembodiments, the separator (or a precursor thereof) has a rock saltcrystal structure, a fluorite crystal structure, or an R3m crystalstructure. It should be understood from the discussion above that theseparator may comprise any of the precursors for the separator. Forinstance, the separator may comprise one or more halide anions, one ormore polyhalide anions, on more or species that may undergo a reactionto form a halide anion and/or a polyhalide anion, chlorate anions,perchlorate anions, nitrate anions, phosphate anions, alkali metalcations, alkaline earth metal cations, and/or halogen atoms as describedabove in reference to the precursors for the separator. In someembodiments, the separator may comprise an alkali halide salt that isdoped with an alkaline earth cation, such as lithium iodide that isdoped with magnesium. Further non-limiting examples of suitableseparator materials include LiF, LiCl, LiBr, LiI, Li₄I₃Br, Li₄Br₃I,Li₄Br₃Cl, Li₄Cl₃Br, and lithium oxyhalides such as Li₃OBr. In someembodiments, one or more lithium oxyhalides may form by abstracting anoxygen from a solvent molecule.

As described above, in certain embodiments a rechargeableelectrochemical cell comprises an electrolyte. In some embodiments, theelectrolyte is a liquid electrolyte. In some embodiments, theelectrolyte may comprise an organic solvent, such as a solventcomprising one or more of an ether group, a nitrile group, a cyanoestergroup, a fluoroester group, a tetrazole group, a fluorosulfonyl group, achlorosulfonyl group, a nitro group, a carbonate group, a dicarbonategroup, a nitrate group, a fluoroamide group, a dione group, an azolegroup, and a triazine group. In some embodiments, the electrolyte maycomprise an alkyl carbonate, such as ethylene carbonate and/ordimethylene carbonate.

In some embodiments, an electrolyte as described herein may comprise oneor more salts to enhance the conductivity of the electrolyte.Non-limiting examples of suitable salts include LiPF₆, LiBF₄, LiFSI,LiTFSI, LiClO₄, LiBOB, and LiDFOB.

In embodiments in which an electrolyte comprises a salt which enhancesthe conductivity of the electrolyte, the salt which enhances theconductivity of the electrolyte may be present at a concentration ofgreater than or equal to 0.01 M, greater than or equal to 0.02 M,greater than or equal to 0.05 M, greater than or equal to 0.1 M, greaterthan or equal to 0.2 M, greater than or equal to 0.5 M, greater than orequal to 1 M, or greater than or equal to 2 M. In some embodiments, thesalt which enhances the conductivity of the electrolyte may be presentat a concentration of less than or equal to 5 M, less than or equal to 2M, less than or equal to 1 M, less than or equal to 0.5 M, less than orequal to 0.2 M, less than or equal to 0.1 M, less than or equal to 0.05M, or less than or equal to 0.02 M. Combinations of the above-referencedranges are also possible (e.g., greater than or equal to 0.01 M and lessthan or equal to 5 M). Other ranges are also possible.

In some embodiments, an electrolyte as described herein may comprise oneor more additives that affects the solubility of a precursor for aseparator in the electrolyte. For example, the electrolyte may comprisean additive that increases the solubility of a precursor for theseparator in the electrolyte. That is, the precursor for the separatormay have a higher solubility in the electrolyte in the presence of theadditive than in an otherwise equivalent electrolyte that lacks theadditive. Non-limiting examples of additives that increase thesolubility of the precursor in the electrolyte include water, HF, andHNO₃. In some embodiments, the additive may comprise one or more of anitrile group, a fluorosulfonyl group, a chlorosulfonyl group, a nitrogroup, a nitrate group, a fluoroamide group, and a dione group.

As described above, certain embodiments relate to rechargeableelectrochemical cells. In some embodiments, the rechargeableelectrochemical cell comprises at least a first electrode. In someembodiments, the first electrode is an anode. The first electrode maycomprise an alkali metal, such as lithium metal, sodium metal, and/orpotassium metal. In some embodiments, the first electrode may comprisean alkaline earth metal, such as magnesium and/or calcium. In someembodiments, the first electrode comprises a transition metal (such asyttrium and/or zinc) and/or a post transition metal (such as aluminum).

A first electrode may have any suitable thickness. In some embodiments,the thickness of the first electrode is greater than or equal to 15microns, greater than or equal to 20 microns, greater than or equal to25 microns, greater than or equal to 30 microns, greater than or equalto 35 microns, greater than or equal to 40 microns, or greater than orequal to 45 microns. In some embodiments, the thickness of the firstelectrode may be less than or equal to 50 microns, less than or equal to45 microns, less than or equal to 40 microns, less than or equal to 35microns, less than or equal to 30 microns, less than or equal to 25microns, or less than or equal to 20 microns. Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 15 microns and less than or equal to 50 microns). Other ranges arealso possible. The thickness of the first electrode may be determined byelectron microscopy.

In some embodiments, a surface of a first electrode may be passivated.For instance, a passivation layer may be disposed on the surface of thefirst electrode. When present, the passivation layer may comprise aprecursor for a separator as described above, such as a halide anion. Insome embodiments, the passivation layer comprises a polymer, such aspoly(2-vinyl pyridine). The passivation layer may be formed by exposingthe surface of the first electrode to a composition comprising a speciesthat reacts with the surface of the first electrode to form thepassivation layer. For example, the surface of the first electrode maybe exposed to a composition comprising a halide salt, such as a lithiumhalide salt. As another example, a passivation layer may be formed byexposing the surface of the first electrode first to poly(2-vinylpyridine) and then to iodine vapor.

When present, a passivation layer on a first electrode may have anysuitable thickness. In some embodiments, the thickness of thepassivation layer on the first electrode is greater than or equal to 1nm, greater than or equal to 2 nm, greater than or equal to 5 nm,greater than or equal to 10 nm, greater than or equal to 20 nm, greaterthan or equal to 50 nm, greater than or equal to 100 nm, greater than orequal to 200 nm, greater than or equal to 500 nm, greater than or equalto 1 micron, greater than or equal to 2 microns, greater than or equalto 5 microns, greater than or equal to 10 microns, greater than or equalto 20 microns, or greater than or equal to 50 microns. In someembodiments, the thickness of the passivation layer on the firstelectrode is less than or equal to 100 microns, less than or equal to 50microns, less than or equal to 20 microns, less than or equal to 10microns, less than or equal to 5 microns, less than or equal to 2microns, less than or equal to 1 micron, less than or equal to 500 nm,less than or equal to 200 nm, less than or equal to 100 nm, less than orequal to 50 nm, less than or equal to 20 nm, less than or equal to 10nm, less than or equal to 5 nm, or less than or equal to 2 nm.Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 1 nm and less than or equal to 50 microns, orgreater than or equal to 10 nm and less than or equal to 1 micron). Thethickness of the passivation layer may be determined by electronmicroscopy.

As described above, certain embodiments relate to rechargeableelectrochemical cells. In some embodiments, the rechargeableelectrochemical cell comprises at least a first electrode and a secondelectrode. In some embodiments, the second electrode is a cathode. Thesecond electrode may comprise one or more of an intercalation compoundsuch as a lithium ion intercalation compound, a conversion compound, anoxide, sulfur, a halide, and a chalcogenide. Non-limiting examples ofsuitable intercalation compounds include lithium cobalt oxide, lithiumiron phosphate, lithium nickel cobalt manganese oxide, and lithiumnickel cobalt aluminum oxide. In some embodiments, the rechargeableelectrochemical cell is a metal-air cell, such as a lithium-air cell.

A second electrode may store a working ion present in the rechargeableelectrochemical cell at any suitable potential. As used herein, aworking ion is a species that is formed by oxidation of a metal speciesat the anode and intercalates into the cathode to provide electricalneutrality to the cathode when the cathode active species is reduced. Insome embodiments, the second electrode stores the working ion at avoltage of greater than or equal to 2.0 V, greater than or equal to 2.5V, greater than or equal to 3.0 V, greater than or equal to 3.5 V,greater than or equal to 4.0 V, or greater than or equal to 4.5 V. Insome embodiments, the second electrode stores the working ion at avoltage of less than or equal to 5.0 V, less than or equal to 4.5 V,less than or equal to 4.0 V, less than or equal to 3.5 V, less than orequal to 3.0 V, or less than or equal to 2.5 V. Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 2.5 V and less than or equal to 5 V). Other ranges are also possible.The voltage at which the working ion is stored may be determined bycyclic voltammetry.

In some embodiments, a rechargeable electrochemical cell may comprise asecond electrode with a thickness of greater than or equal to 20microns, greater than or equal to 50 microns, greater than or equal to100 microns, or greater than or equal to 200 microns. In someembodiments, a rechargeable electrochemical cell may comprise a secondelectrode with a thickness of less than or equal to 500 microns, lessthan or equal to 200 microns, less than or equal to 100 microns, or lessthan or equal to 50 microns. Combinations of the above-referenced rangesare also possible (e.g., greater than or equal to 20 microns and lessthan or equal to 50 microns). Other ranges are also possible. Thethickness of the second electrode may be determined by electronmicroscopy.

In some embodiments, the first electrode and/or the second electrode maybe disposed on a current collector. The current collector may be inelectrical communication with the electrode disposed on it, and may becapable of transmitting electrons from the electrode to an externalcomponent (e.g., a load). Non-limiting examples of suitable currentcollectors include copper, nickel, aluminum, titanium, chrome, graphite,and glassy carbon. The current collector may be in the form of a foil, amesh, and/or a foam.

In some embodiments, a rechargeable electrochemical cell may compriseone or more ex situ separator(s), or a separator(s) that are added tothe cell during the cell assembly process. The ex situ separator(s) mayhave any suitable composition and structure. Non-limiting examples ofsuitable ex situ separators include porous materials, such as porouspolymer membranes (e.g., cellulosic membranes), porous ceramicmembranes, fiber mats (e.g., glass fiber mats), woven structures, and/ornon-woven structures. In some embodiments, one or more ex situseparators present in a rechargeable electrochemical cell may comprise acoating. When more than one ex situ separator is present, each ex situseparator may independently have any, all, or none of the propertiesdescribed herein.

In some embodiments, a rechargeable electrochemical cell as describedherein may have one or more advantageous properties, such as being freefrom dendrites after cycling. In some embodiments, the electrochemicalcell may be free from dendrites after greater than or equal to 50cycles, greater than or equal to 100 cycles, greater than or equal to200 cycles, greater than or equal to 500 cycles, greater than or equalto 1000 cycles, or greater than or equal to 3000 cycles. In someembodiments, the electrochemical cell may be free from dendrites afterless than or equal to 5000 cycles, less than or equal to 3000 cycles,less than or equal to 1000 cycles, less than or equal to 500 cycles,less than or equal to 200 cycles, or less than or equal to 200 cycles.Combinations of the above-referenced ranges are also possible (e.g.,greater than or equal to 50 cycles and less than or equal to 5000cycles). Other ranges are also possible.

In some embodiments, a rechargeable electrochemical cell may have arelatively high area capacity. The area capacity of the rechargeableelectrochemical cell may be greater than or equal to 2 mAh/cm², greaterthan or equal to 3 mAh/cm², greater than or equal to 4 mAh/cm², greaterthan or equal to 5 mAh/cm², greater than or equal to 6 mAh/cm², greaterthan or equal to 7 mAh/cm², greater than or equal to 8 mAh/cm², greaterthan or equal to 9 mAh/cm², greater than or equal to 10 mAh/cm², greaterthan or equal to 11 mAh/cm², greater than or equal to 12 mAh/cm²,greater than or equal to 13 mAh/cm², greater than or equal to 14mAh/cm², greater than or equal to 15 mAh/cm², greater than or equal to16 mAh/cm², greater than or equal to 17 mAh/cm², greater than or equalto 18 mAh/cm², or greater than or equal to 19 mAh/cm². The area capacityof the rechargeable electrochemical cell may be less than or equal to 20mAh/cm², less than or equal to 19 mAh/cm², less than or equal to 18mAh/cm², less than or equal to 17 mAh/cm², less than or equal to 16mAh/cm², less than or equal to 15 mAh/cm², less than or equal to 14mAh/cm², less than or equal to 13 mAh/cm², less than or equal to 12mAh/cm², less than or equal to 11 mAh/cm², less than or equal to 10mAh/cm², less than or equal to 9 mAh/cm², less than or equal to 8mAh/cm², less than or equal to 7 mAh/cm², less than or equal to 6mAh/cm², less than or equal to 5 mAh/cm², less than or equal to 4mAh/cm², or less than or equal to 3 mAh/cm². Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 2 mAh/cm² and less than or equal to 20 mAh/cm², or greater than orequal to 3 mAh/cm² and less than or equal to 10 mAh/cm²). Other rangesare also possible.

In some embodiments, a rechargeable electrochemical cell may have arelatively large cycle life. The cycle life of the rechargeableelectrochemical cell may be greater than or equal to 50 cycles, greaterthan or equal to 100 cycles, greater than or equal to 200 cycles,greater than or equal to 500 cycles, greater than or equal to 1000cycles, or greater than or equal to 3000 cycles. The cycle life of therechargeable electrochemical cell may be less than or equal to 5000cycles, less than or equal to 3000 cycles, less than or equal to 1000cycles, less than or equal to 500 cycles, less than or equal to 200cycles, or less than or equal to 100 cycles. Combinations of theabove-referenced ranges are also possible (e.g., greater than or equalto 50 cycles and less than or equal to 5000 cycles). Other ranges arealso possible.

In some embodiments, a rechargeable electrochemical cell may have arelatively large cycle life in comparison to an otherwise equivalentelectrochemical cell lacking the separator and/or lacking the precursorfor the separator. The cycle life of the rechargeable electrochemicalcell may be greater than or equal to 20% larger than an otherwiseequivalent electrochemical cell, greater than or equal to 50% largerthan an otherwise equivalent electrochemical cell, greater than or equalto 100% larger than an otherwise equivalent electrochemical cell,greater than or equal to 200% larger than an otherwise equivalentelectrochemical cell, greater than or equal to 500% larger than anotherwise equivalent electrochemical cell, or greater than or equal to1000% larger than an otherwise equivalent electrochemical cell. Thecycle life of the rechargeable electrochemical cell may be less than orequal to 2000% larger than an otherwise equivalent electrochemical cell,less than or equal to 1000% larger than an otherwise equivalentelectrochemical cell, less than or equal to 500% larger than anotherwise equivalent electrochemical cell, less than or equal to 200%larger than an otherwise equivalent electrochemical cell, less than orequal to 100% larger than an otherwise equivalent electrochemical cell,or less than or equal to 50% larger than an otherwise equivalentelectrochemical cell. Combinations of the above-referenced ranges arealso possible (e.g., greater than or equal to 20% and less than or equalto 2000% larger than an otherwise equivalent electrochemical cell).Other ranges are also possible.

Example 1

In the example, a combined experimental/computational investigation isdescribed to investigate electrochemical formation of lithium halidebased solid electrolytes, with the goal of enabling and demonstratingself-assembling/self-healing batteries using lithium metal negativeelectrodes. The halogen series (I, Br, Cl and F) exhibits progressivelyhigher reaction potentials with lithium metal. The absolute values ofthese potentials allow controlled formation of lithium halides (or otheralkali halides) at cell potentials that are compatible with a broadrange of electrochemical couples, including lithium-sulfur andlithium-intercalation cathode couples. For example, in the Li—S examplereferenced above, LiI has a formation potential (2.85V) higher than theLi—S reaction (2.5V), corresponding to a more negative free energy offormation, hence it is expected that the SEI on the lithium metal willcontain LiI. In a hypothetical example where Li metal anode were to beused with LiFePO₄ cathode at cell voltage ˜3.45V, LiI solid electrolytewould not spontaneously form, but other halides such as LiBr, LiCl andLiF would. It is believed that mixed halides Li(I,Br,Cl,F) compositions,including graded solid electrolytes, can be produced by appropriatecontrol of the reaction pathway. Each of these metal halidespreferentially crystallize in the rocksalt structure, which is furtheramenable to compositional tuning of its transport properties. Forexample, supervalent cation doping, e.g., with alkaline earth iodides,can produce charge-compensating cation vacancies in the rocksalt latticethat enhance lithium ion conductivity. Strategies for doping of theself-assembling lithium halide solid electrolytes can be evaluatedthrough combined computation and experiment.

Self-healing functionality can be introduced by tuning the metal halideand the liquid electrolyte compositions to provide limited solubility ofthe halide in the liquid electrolyte. Lithium metal that is exposedduring battery cycling, for instance through cracking of the solidelectrolyte film, may be able to be passivated upon exposure to theliquid. The solubility of lithium halides in nonaqueous solvents varies,from high solubilities in ethereal solvents to low solubility in othersolvents. The wide range of formation potentials suggests that thechoice of solvent and halogen allows a wide range of tuning toaccomplish partial solubility. This area is especially amenable tothermodynamics-based computational search in order to guide experiments.

This example investigates the controlled electrochemical formation oflithium-halide based solid electrolytes, with the goal of enabling anddemonstrating self-assembling/self-healing batteries using lithium metalnegative electrodes (see FIG. 6). The relevant performance features fora solid electrolyte compatible with lithium metal electrodes are asfollows:

1. High ionic conductivity of the order of ˜10⁻² S/cm2. Electrochemically stable up to operating potentials of 4-5 V3. High electronic conductivity, typically a material with band gaplarger than 4 eV4. Mechanically strong to suppress dendritic growth, with a shearmodulus >20 GPa5. Thermally stable with a small linear coefficient of thermalexpansion, α<0.00001056. High degree of selectivity (e.g., single-ion conductor)7. Low processing cost8. Ease of integration into existing battery designs

It should be noted that solid electrolytes may have any of thesefeatures, all of these features, or none of these features.

Using these metrics, the radar chart derived from certain lithium halidesalts is presented in FIG. 7. It can be seen that for the case of purehalides, all the metrics are satisfied reasonably well except for ionicconductivity. However, the ionic conductivity is also reasonable for thedoped LiI case.

In this experiment, the ionic conductivity may be improved bycontrolling the composition, including doping, of the in-situ formedsolid halide electrolyte film. With respect to mechanical properties,the halides are high modulus inorganic solids with the ability toprevent Li dendrite penetration, as illustrated in FIG. 6. However,these compounds may have a low fracture toughness. It may be possible toovercome this limitation by developing systems that are self-healing,such that exposure of Li metal that occurs during lithium metalstripping and deposition may be spontaneously and controllablypassivated by new metal halide.

The ability to repair damage spontaneously, by, e.g., self-healing, maybe desirable for rechargeable batteries because electrochemicalreactions in battery materials may result in structural changes (seeFIG. 8). These changes may cause degradation and damage and/orultimately cause the battery to become non-functional with cycling.Applying self-healing halides-based chemistry to lithium metalelectrodes to increase their cycle-life and safety, and to reducedendritic formation. This may be accomplished by one or more of thefollowing strategies:

1. Initial Passivation by Chemical Means: There are numerous ways offorming an initial halide layer on Li metal, amongst the simplest ofwhich may be dipping in a nonaqueous solvent containing a highconcentration of Li⁺ and X⁻ ions. Other methods include exposure tohalogen vapor is another method, and deposition of apoly-2-vinylpyridine film followed by reaction with I₂ (see FIG. 9)2. Electrochemical Control of Solid Halide Composition: At the initiallypassivated surface, Li(X,Y) halides may then be deposited via control ofthe applied potential and speciation of the halides species in solution.All components are additives may be in the liquid electrolyte and theformation process may be carried out on the assembled cell. As shownabove, deposition of the metal halides occurs with increasing potentialin the order I, Br, Cl and F. Thus a thermodynamics-basedchronopotentiometric profile and the dissolved halides composition maywork together to determine the composition of the solid halide film.Electrochemical deposition mediated by another halogen ion in thesolution may also be effective for the formation of conformal coatingsof halides. For example, an LiCl layer can be mediated by a Br⁻ ionpresent in the solution phase.

The scheme may proceeds in the following sequence of steps:

a. Formation of a Mixed Halide Anion:

X₂+Y⁻→X₂Y

b. Anodic Reaction:

Li→Li⁺ +e ⁻

c. Cathodic Reaction:

2Li++X₂Y+2e ⁻→2LiX+Y⁻

In the above scheme, the formation of LiX may be mediated by Y⁻ ions.Note that the anode need not be lithium metal but could also be anyother material that oxidation releases Li⁺ ions in solution; forexample, the halide solid electrolyte could also be deposited on acathode.

3. Controlling Solubility of Halides for Self-Healing: The formationpotentials of all the halides are large enough that exposed Li metal maybe spontaneously passivated by at least a monolayer of LiX. If the LiXis completely insoluble in the electrolyte, then a substantial fractionor all X in solution may eventually be irreversibly “gettered” byexposed Li metal, and the ability of the solid iodide to passivate newlyexposed Li metal against dendrite formation may eventually cease. Thusproviding a persistent source of dissolved halides for self-healing ofdefects formed in a halide solid electrolyte during battery cycling maybe advantageous. Given the wide range of formation potential for thehalides, and the known large variation in LiI solubility amongstnonaqueous solvents, a wide “window” for tuning halide solubility in theelectrolyte exists. Qualitatively, the solubility of LiX in any solventwill decrease in the order X═I, Br, Cl, F. In this part of the project,computations to calculate the dissolution free energy based on theadsorption energy, donor number and acceptor number of the electrolytemay guide experimental designs to systematically map the solubility ofhalides as a function of solvent type, such as alkyl carbonate solvents.Combined with experimental characterization of the LiX layer that formson the Li metal and the concentration and speciation of halide in theelectrolyte, it may be possible to map the partitioning of halidebetween the solid electrolyte layer and the equilibrium solubility inthe electrolyte. A suitable thermodynamic description is that adsorptionof halide species from liquid solution at sites of high Li activity mayoccur, and it may be beneficial to maintain an adequate source ofhalides in the liquid for self-healing and/or to control the compositionand structure of the adsorbed layers that form on lithium metal.

The stability of the passivated Li metal surface against dendriteformation may be determined for relevant electrokinetic parameters suchas the current density and thickness/capacity of lithium metal that isreversibly plated. It may be desirable to suppress dendrite formation atarea capacities of 3 to 10 mAh/cm², corresponding to Li metal thicknessof 15 to 50 μm.

It is recognized that the following technical barriers may beencountered:

1. Obtaining high ionic conductivity: Pure halides may have a relativelylow ionic conductivity. Preliminary computational results indicate thatalkaline earth doping, such as with Mg²⁺, may increase the ionicconductivity of the halide salt.2. Selective formation of solid electrolyte at the lithium interface:Tuning the solvent and salt anion chemistry such that the desiredsolution species and solid electrolyte film can be selectively formed.3. Mitigating dendrite growth: Pure and mixed halides both showpromising mechanical properties for the suppression of dendrite growth.Achieving selectivity in electrolyte film growth will allow thethickness and mechanical properties to be tuned for prevention ofdendrites.4. Achieving self-healing response (see FIG. 10): Defects in the solidhalide electrolyte film may spontaneously passivate with new solidelectrolyte rapidly enough to prevent preferential deposition of new Limetal leading to dendrite formation.

Multiple aspects including nucleation of lithium halides, stability andgrowth on lithium metal, and the mechanical strength have beeninvestigated in order to confirm the feasibility of lithium halides asstable solid electrolytes. Since adsorption energies of halides onlithium metal determine the operating-potential range for the nucleationof halides, obtaining adsorption energies of halides using DensityFunctional Theory (DFT) may provide relevant insight. Adsorptionenergies are calculated by running DFT simulations with halides asadsorbed molecules on a four-atom-thick lithium slab. Threeconfigurations—on top, bridge and hollow positions—are simulated and thelowest energy configuration is used for calculating the adsorptionenergy. The adsorption energies are obtained as the formation energiesof the adsorbed states, for which the energy of the lithium slab aloneis obtained from DFT beforehand. FIG. 11 shows the adsorption andformation energies of the LiF, LiCl, LiBr, LiI. For the case of polyhalides and mixed halides, the adsorbed molecule is simulated similarlyby replacing it with a molecule of the respective mixed/poly halide.

Solid electrolytes have the potential to significantly improving thesafety and energy density of batteries. Solid electrolytes with suitablemechanical properties may suppress the growth of dendrites on lithiummetal anodes, whose use can greatly enhance the energy density of thebattery. Both of these perspectives call for carefully studying themechanical properties of solid electrolytes.

Using DFT, the following formalism is used to calculate the elasticmoduli and mechanical properties of lithium and mixed halides. Thegeneral stress-strain constitutive relation for anisotropic materials isused, wherein the stress is related to strain by an elastic 6 x 6tensor. Using density functional theory calculations of the strainedhalide structures, the stress is calculated at a series of appliedstrains. The stresses and strains are then fitted to the generalstress-strain relationship, and the components of the elastic tensor arerecovered from the fitting parameters. Table 1, below, shows elasticmoduli of lithium halides calculated using density functional theory.

TABLE 1 Halide C₁₁ (GPa) C₂₁ (GPa) C₄₄ (GPa) B (GPa) G (GPa) LiF 175.8757.99 66.12 97.28 70.57 LiCl 71.13 27.79 29.81 42.24 28.96 LiBr 56.2922.61 23.29 33.83 22.57 LiI 46.13 17.39 18.12 26.97 18.33 Li₄I₃Br 47.7218.36 18.76 28.14 18.86 Li₄Br₃I 53.05 20.69 21.09 31.47 21.01 Li₄Br₃Cl59.25 23.42 24.48 35.37 23.86 Li₄Cl₃Br 66.57 26.24 27.57 39.69 26.86

As a criterion to investigate the feasibility of halides for thesuppression of dendrites the deformation chemical potential is employed.The modified Butler-Volmer model with interfacial deformation is used:

$i = {{i_{0,{ref}}{{\exp \lbrack \frac{( {1 - \alpha_{a}} )\Delta \; \mu_{e^{-}}}{RT} \rbrack}\lbrack {\exp ( \frac{\alpha_{a}F\; \eta_{s}}{RT} )} \rbrack}} - \lbrack {\exp ( {- \frac{\alpha_{c}F\; \eta_{s}}{RT}} )} \rbrack}$

Based on the response of the electrode-electrolyte interface to aperiodic displacement u(x₁, 0)=A cos ωx₁ along the interface, thedeformation and compressive forces experienced by the electrolytesurface will be calculated. This will help determine the value ofoptimal range of mechanical properties required to suppress dendritegrowth.

The schemes mentioned in the section on relevance and outcomes provide aneat solution of forming a single component SEI layer on the electrode.However, owing to the complex nature of polyhalide chemistry, it may bebeneficial to control the potential and concentration of the species inthe electrolyte precisely so that only the desired lithium halide (LiX)is formed. Depending on the concentration of X⁻ and dissolved X₂/Y₂ itis possible to form multiple ionic species such as X⁻, X₃ ⁻, XY₂ ⁻, andXY⁻². Thus, it may be beneficial to consider the equilibrium for thefollowing reactions in various solvents:

X⁻+X₂

X₃ ⁻

X⁻+X₂

*X₃ ⁻

X⁻+X₂

X₃ ⁻

It may also be beneficial to consider the formation of the followingcomplex ionic species through the electrochemical pathways shown below:

X⁻+X₂

X₃ ⁻

X⁻+X₂

X₃ ⁻

The formation of even larger polyhalide complexes is also possible butmay require large cations for stabilization. Considering that Li⁺ is avery small cation, the existence of complexes with more than 3 halideatoms is improbable. Using the potentials for the above electrochemicalreactions, it may be possible to develop a solution phase diagram ofhalide species at different potentials. An example of such a solutionphase diagram for I⁻/I₂/I₃ ⁻ in different solvents is shown in FIG. 12.

Another aspect to consider for enabling the self-healing function ispartial dissolution of LiX in the electrolyte. The dissolution reactionis given by:

LiX

Li⁺+X⁻

The free energy change associated with this reaction is ΔG_(diss)=G_(Li)₊ +G_(X) ⁻ −G_(LiX) where G_(Li) ₊ depends on the Gutmann donor numberof the solvent and the concentration of Li⁺, G_(X) ⁻ depends on theGutmann acceptor number of the solvent and the concentration of X⁻ andG_(LiX) is calculated from Density Functional Theory (DFT) calculations.For the purpose of forming a stable SEI, it may be desirable to minimizethe dissolution reaction and for enabling the self-healing reaction, itmay be desirable to have partial dissolution. This may be possible bymaking ΔG_(diss)˜0 by choosing the appropriate lithium halide andelectrolyte combination.

Preliminary results are shown in FIGS. 13A-C. Here, symmetric Li—Licells have been subjected to cycling at 5 mA/cm² current density thatreversibly strips and plates 10 μm of Li metal (2.05 mAh/cm²). The twoLi electrodes are separated by a Whatman glass fiber mat, and theelectrolyte is 1M LiPF₆ in EC:DMC (1:1), with and without 0.1 M additionof LiI. Upon cycling, it was observed that the cells with LiI additiveconsistently showed both a lower initial impedance (FIG. 13A) as well asa lower impedance growth rate (FIG. 13B) than the cells without LiI.After about 70 cycles, the cells without LiI are exhibiting runawayimpedance growth, whereas the cells with LiI are still cycling stably.EIS conducted after 70 cycles (FIG. 13C) shows a low frequency arc ofmuch higher impedance in the case without LiI additive. These resultsare consistent with the rapid formation of an LiI-based solidelectrolyte film of low resistance on the Li metal electrode, which thenremains stable through extensive cycling. These results suggest improvedstability of the Li electrode upon using LiI and that there may be muchroom for discovery upon using the proposed mixed LiX halides.

In order to discern the halide and polyhalide species under operatingconditions, two-electrode measurements using carbon paper as the workingelectrode, Li metal as the counter/quasi-reference electrode werecarried out at a scan rate of 5 mV/s. Based on the anodic scan data, itis seen that the relevant species are LiI, LiI₃, and I₂ (FIG. 14A). Thisis in good agreement with theoretical analysis for the dominant speciesshown in FIG. 14B. In order to show the effect of halide in tuning thedissolution potential, the comparison between LiI and LiBr in shown inFIG. 14C. In this case, there is a positive shift of the oxidationpotential for the halide anions. In order to demonstrate the possibilityfor the formation of polyhalide species as discussed above, the cyclicvoltammetry profile of the cell containing mixed halide (0.5 M LiI andLiBr) is shown in FIG. 14D and it is seen that speciation may bedifferent from those of LiI-containing or LiBr containing cells. Thesesuggest the formation of mixed polyhalide species and based on thisdata, it is believed that there may be evidence that the proposedself-forming and self-healing halide-based scheme is viable.

The results presented so far for the lithium halides scheme show that itcan potentially solve the dendrite growth problem. To summarize, theproposed lithium halides scheme may have one or more advantages:

1. The self-forming process to make the protected electrode may besimple and scalable.2. The mixed halide based formation process may be a step towardsenabling three dimensional electrodes, which can enable very high energydensity owing to a much higher surface area in comparison to the twodimensional electrodes.3. The self-formed protected lithium electrode may not suffer from theissues faced by other additive approaches.4. The self-healing function may be a unique aspect for the proposedscheme that has not been successfully and reliably demonstrated earlier.

Computational research may be used to compute the thermodynamics ofpolyhalide speciation to determine the stability range for these variouspolyhalide species under the operating potential. Polyhalides havecomplex chemistry and can form X⁻, X₃ ⁻, X₅ ⁻, X₇ ⁻, X₄ ²⁻, etc and thefree energies may be calculated using first-principles densityfunctional theory calculations within an implicit solvation framework.The thermodynamics of polyhalides may change depending on the solvent.This analysis may provide an understanding of the active polyhalidespecies that will enable the self-assembling and self-healing processesand may be supplemented by half-cell and full cell testing.

Experimentally, the proposed self-assembling, self-limiting solidelectrolytes may initially be formed on lithium metal via metal halideadditives to liquid electrolytes. A variety of two-electrode andthree-electrode cell constructions may be used to systematically isolateand interrogate formation of solid halide films on lithium metal. Celldesigns may include “half-cells” having a lithium working electrode andnonreactive metal counter-electrode, symmetric lithium-lithium cells,and “full cells” including Li—S and Li-intercalation cathode cells.Based on laboratory cell testing, down-selects may be performed andprototype full cells of >10 mAh capacity will be fabricated anddelivered to DOE-specified laboratories for testing and evaluation.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” can refer,in one embodiment, to A only (optionally including elements other thanB); in another embodiment, to B only (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

It should also be understood that, unless clearly indicated to thecontrary, in any methods claimed herein that include more than one stepor act, the order of the steps or acts of the method is not necessarilylimited to the order in which the steps or acts of the method arerecited.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

Any terms as used herein related to shape, orientation, alignment,and/or geometric relationship of or between, for example, one or morearticles, structures, forces, fields, flows, directions/trajectories,and/or subcomponents thereof and/or combinations thereof and/or anyother tangible or intangible elements not listed above amenable tocharacterization by such terms, unless otherwise defined or indicated,shall be understood to not require absolute conformance to amathematical definition of such term, but, rather, shall be understoodto indicate conformance to the mathematical definition of such term tothe extent possible for the subject matter so characterized as would beunderstood by one skilled in the art most closely related to suchsubject matter. Examples of such terms related to shape, orientation,and/or geometric relationship include, but are not limited to termsdescriptive of: shape—such as, round, square, circular/circle,rectangular/rectangle, triangular/triangle, cylindrical/cylinder,elliptical/ellipse, (n)polygonal/(n)polygon, etc.; angularorientation—such as perpendicular, orthogonal, parallel, vertical,horizontal, collinear, etc.; contour and/or trajectory—such as,plane/planar, coplanar, hemispherical, semi-hemispherical, line/linear,hyperbolic, parabolic, flat, curved, straight, arcuate, sinusoidal,tangent/tangential, etc.; direction—such as, north, south, east, west,etc.; surface and/or bulk material properties and/or spatial/temporalresolution and/or distribution—such as, smooth, reflective, transparent,clear, opaque, rigid, impermeable, uniform(ly), inert, non-wettable,insoluble, steady, invariant, constant, homogeneous, etc.; as well asmany others that would be apparent to those skilled in the relevantarts. As one example, a fabricated article that would described hereinas being “square” would not require such article to have faces or sidesthat are perfectly planar or linear and that intersect at angles ofexactly 90 degrees (indeed, such an article can only exist as amathematical abstraction), but rather, the shape of such article shouldbe interpreted as approximating a “square,” as defined mathematically,to an extent typically achievable and achieved for the recitedfabrication technique as would be understood by those skilled in the artor as specifically described. As another example, two or more fabricatedarticles that would described herein as being “aligned” would notrequire such articles to have faces or sides that are perfectly aligned(indeed, such an article can only exist as a mathematical abstraction),but rather, the arrangement of such articles should be interpreted asapproximating “aligned,” as defined mathematically, to an extenttypically achievable and achieved for the recited fabrication techniqueas would be understood by those skilled in the art or as specificallydescribed.

What is claimed is:
 1. A method of forming a separator in a rechargeableelectrochemical cell, comprising: holding a first electrode at a firstvoltage in a rechargeable electrochemical cell, wherein the rechargeableelectrochemical cell comprises an electrolyte comprising a precursor forthe separator in an amount of less than or equal to 1 mM and greaterthan or equal to 1 nM, and wherein the first voltage causes theprecursor for the separator to react to form a separator positionedbetween the first electrode and a second electrode.
 2. A method ofhealing a defect in a separator in a rechargeable electrochemical cell,comprising: holding a first electrode at a first voltage in arechargeable electrochemical cell, wherein the rechargeableelectrochemical cell comprises a separator, wherein the rechargeableelectrochemical cell comprises an electrolyte comprising a precursor forthe separator in an amount of less than or equal to 1 mM and greaterthan or equal to 1 nM, and wherein the first voltage causes theprecursor for the separator to react to heal a defect in the separator.3. A rechargeable electrochemical cell, comprising: a first electrode; asecond electrode; and an electrolyte, wherein the electrolyte comprisesa precursor for a separator, and wherein the precursor for the separatorhas a solubility in the electrolyte of less than or equal to 1 mM andgreater than or equal to 1 nM.
 4. A rechargeable electrochemical cell,comprising: a first electrode; a second electrode; and an electrolyte,wherein the electrolyte comprises at least one of a first halide anionand a species that can react to form a first halide anion, and whereinthe electrolyte comprises at least one of a second halide anion and aspecies that can react to form a second halide anion.
 5. A rechargeableelectrochemical cell, comprising: a first electrode; a second electrode;and a separator, wherein the separator comprises at least a first layerand a second layer, and wherein the second layer undergoes oxidation ata higher voltage than the first layer.
 6. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the precursorfor the separator is capable of healing a defect in the separator.
 7. Amethod or rechargeable electrochemical cell as in any preceding claim,wherein the precursor for the separator comprises a halide anion.
 8. Amethod or rechargeable electrochemical cell as in claim 7, wherein thehalide anion is one of a fluoride anion, a chloride anion, a bromideanion, and an iodide anion.
 9. A method or rechargeable electrochemicalcell as in any preceding claim, wherein the precursor for the separatorcomprises a species that can react to form a halide anion.
 10. A methodor rechargeable electrochemical cell as in any preceding claim, whereinthe precursor for the separator comprises a polyhalide anion.
 11. Amethod or rechargeable electrochemical cell as in any preceding claim,wherein the polyhalide anion comprises at least two halogen species. 12.A method or rechargeable electrochemical cell as in any preceding claim,wherein the precursor for the separator comprises a halogen with anoxidation state of
 0. 13. A method or rechargeable electrochemical cellas in any preceding claim, wherein the precursor for the separatorcomprises at least one of a chlorate anion, a perchlorate anion, anitrate anion, a phosphate anion, a PF₆ ⁻ anion, a BF₄ ⁻ anion, abis(fluorosulfonyl)imide anion, and a bis(trifluoromethane)sulfonimideanion.
 14. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the precursor for the separator comprises alithium cation.
 15. A method or rechargeable electrochemical cell as inany preceding claim, wherein the precursor for the separator comprisesan alkaline earth metal cation.
 16. A method or rechargeableelectrochemical cell as in claim 15, wherein the alkaline earth metalcation is Mg²⁺.
 17. A method or rechargeable electrochemical cell as inany preceding claim, wherein the precursor for the separator comprisesat least two halide anions.
 18. A method or rechargeable electrochemicalcell as in any preceding claim, wherein the precursor for the separatordoes not participate in a redox shuttle when the rechargeableelectrochemical cell operates at a voltage of greater than or equal to 4V.
 19. A method or rechargeable electrochemical cell as in any precedingclaim, wherein the precursor for the separator has a rock salt crystalstructure.
 20. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the precursor for the separator has a fluoritecrystal structure.
 21. A method or rechargeable electrochemical cell asin any preceding claim, wherein the precursor for the separatorcrystallizes in a R3m crystal structure.
 22. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the electrolytecomprises a solvent with one or more of an ether group, a nitrile group,a cyanoester group, a fluoroester group, a tetrazole group, afluorosulfonyl group, a chlorosulfonyl group, a nitro group, a carbonategroup, a dicarbonate group, a nitrate group, a fluoroamide group, adione group, an azole group, and a triazine group.
 23. A method orrechargeable electrochemical cell as in any preceding claim, wherein theelectrolyte comprises an alkyl carbonate solvent.
 24. A method orrechargeable electrochemical cell as in any preceding claim, wherein theelectrolyte comprises an additive that increases the solubility of theprecursor for the separator in the electrolyte.
 25. A method orrechargeable electrochemical cell as in any preceding claim, wherein asolubility of the precursor for the separator in the electrolyte isgreater than or equal to 1 nM and less than or equal to 1 mM.
 26. Amethod or rechargeable electrochemical cell as in any preceding claim,wherein the precursor for the separator is present in the electrolyte ata concentration of greater than or equal to 1 nM and less than or equalto 1 mM.
 27. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the electrolyte additive comprises one or moreof a nitrile group, a fluorosulfonyl group, a chorosulfonyl group, anitro group, a nitrate group, a fluoroamide group, and a dione group.28. A method or rechargeable electrochemical cell as in any precedingclaim, wherein the ionic conductivity of the separator is greater thanor equal to 10⁻⁴ S/cm.
 29. A method or rechargeable electrochemical cellas in any preceding claim, wherein the area-specific impedance of theseparator is less than or equal to 20 Ohm*cm².
 30. A method orrechargeable electrochemical cell as in any preceding claim, wherein theseparator is stable up to an operating potential of 3.7 V.
 31. A methodor rechargeable electrochemical cell as in any preceding claim, whereinthe separator has a shear modulus of greater than or equal to 5 GPa. 32.A method or rechargeable electrochemical cell as in any preceding claim,wherein the separator has a shear modulus of greater than or equal to 20GPa.
 33. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the separator has a coefficient of thermalexpansion of less than or equal to 0.0000105 K⁻¹.
 34. A method orrechargeable electrochemical cell as in any preceding claim, wherein theseparator is a single ion conductor.
 35. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the separator isdirectly adjacent to the first electrode.
 36. A method or rechargeableelectrochemical cell as in any preceding claim, wherein a thickness ofthe separator is greater than or equal to 1 nm and less than or equal to50 microns.
 37. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the separator comprises at least a first layerand a second layer.
 38. A method or rechargeable electrochemical cell asin any preceding claim, wherein the first layer has a differentcomposition than the second layer.
 39. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the second layerundergoes oxidation at a higher voltage than the first layer.
 40. Amethod or rechargeable electrochemical cell as in any preceding claim,wherein the first layer is closer to the first electrode than the secondlayer.
 41. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the separator comprises at least a third layer.42. A method or rechargeable electrochemical cell as in any precedingclaim, wherein a surface of the first electrode is passivated.
 43. Amethod or rechargeable electrochemical cell as in any preceding claim,wherein a surface of the first electrode comprises a passivation layer.44. A method or rechargeable electrochemical cell as in any precedingclaim, wherein the passivation layer has a thickness of greater than orequal to 1 nm and less than or equal to 100 microns.
 45. A method orrechargeable electrochemical cell as in any preceding claim, wherein thepassivation layer comprises a halide anion.
 46. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the passivationlayer comprises poly(2-vinyl pyridine).
 47. A method or rechargeableelectrochemical cell as in any preceding claim, wherein a thickness ofthe first electrode is greater than or equal to 15 microns and less thanor equal to 50 microns.
 48. A method or rechargeable electrochemicalcell as in any preceding claim, wherein the second electrode stores aworking ion of the rechargeable electrochemical cell at a potentialrelative to the metal of the working ion that is greater than 2.0 V. 49.A method or rechargeable electrochemical cell as in any preceding claim,wherein the second electrode comprises at least one of an intercalationcompound, a conversion compound, an oxide, a halide, and achalcogenide.—wherein the second electrode comprises a lithium ionintercalation compound.
 50. A method or rechargeable electrochemicalcell as in any preceding claim, wherein the second electrode comprisesat least one of lithium cobalt oxide, lithium iron phosphate, lithiumnickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide,sulfur, and air.
 51. A method or rechargeable electrochemical cell as inany preceding claim, wherein the first electrode comprises lithiummetal.
 52. A method or rechargeable electrochemical cell as in anypreceding claim, wherein the first electrode comprises sodium metal. 53.A method or rechargeable electrochemical cell as in any preceding claim,wherein the first electrode comprises potassium metal.
 54. A method orrechargeable electrochemical cell as in any preceding claim, wherein thefirst electrode comprises one or more of magnesium, calcium, yttrium,zinc, and aluminum.
 55. A method or rechargeable electrochemical cell asin any preceding claim, wherein the rechargeable electrochemical cellfurther comprises an ex situ separator.
 56. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the firstvoltage is greater than or equal to 3 V and less than or equal to 5 V.57. A method or rechargeable electrochemical cell as in any precedingclaim, wherein the rechargeable electrochemical cell is free fromdendrites after greater than or equal to 50 cycles.
 58. A method orrechargeable electrochemical cell as in any preceding claim, wherein therechargeable electrochemical cell has an area capacity of greater thanor equal to 3 mAh/cm² and less than or equal to 10 mAh/cm².
 59. A methodor rechargeable electrochemical cell as in any preceding claim, whereinthe rechargeable electrochemical cell has a cycle life of greater thanor equal to 50 cycles.
 60. A method or rechargeable electrochemical cellas in any preceding claim, wherein the rechargeable electrochemical cellhas a cycle life of greater than or equal to 20 larger than the cyclelife of an otherwise equivalent electrochemical cell lacking theprecursor for the separator.
 61. A method or rechargeableelectrochemical cell as in any preceding claim, wherein the separator isa solid electrolyte